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Why do most commercial airplanes use hydraulics instead of electric/servo motors?
I'm sure it's possible to use servo motors to get precise control of the ailerons, elevators or the rudder. But still, hydraulics are widely used instead.
Hydraulics do have a disadvantage that a damage to the hydraulic pipes can cause loss of control. Even with redundancy, there are cases where all three hydraulic lines get damaged at the same time and there's complete loss of control (can't remember the example). I think in some smaller aircrafts servo motors are used. But why not in the larger ones. Is it because of the torque required?
What are the issues with using electric motors?
If used what are the safety concerns?
In terms of safety, how different are they from hydraulics?

Electrical options are mentioned in the hydraulics vs. pneumatics question.

Check out this white paper on the topic

Electric motors are usually very heavy, requiring quite a bit of copper and magnetic material to operate. On top of that, servo motors typically require a lot of power, especially when being operated quickly like ailerons, rudders, and elevators need to be. That being said, there are servo-hydraulics (servo valves, hydraulic actuators) in aircraft.

Electricity also can lead to fires (e.g. if the electric lines are severed -that can happen, too- and sparks contact fuel)

What fluids are used in aviation hydraulics?

I'm sure it's possible to use servo motors to get precise control of the ailerons, elevators or the rudder.

No, it isn't. Primary flight control actuators require both rather high forces and quick response time. Electric motors can provide either, but both at the same time is a problem. Hydraulics is still better for that combination.
Note that other actuators that don't require the fast response time (like flaps, gear or horizontal stabilizer foreplane (elevator trim)) are electric in some aircraft.

Is it possible that fewer hydraulic motors are required because they can be shared throughout the plane? I would think each surface would need it's own servo-motor (or linkages).

Doesn't the 787 use an electrical aileron control package though (with hydraulic backup)?

@shortstheory, I suspect you are confusing fly-by-wire with the actual actuation. The system is fly-by-wire, meaning that control column is sensed electrically and computer processes the commands to the control surfaces, but the actuation can still be hydraulic. The computer uses solenoids, that have the precision and fast response, to control hydraulic valves, which does not need much power, and uses hydraulics to provide the forces. I have not studied 787 systems, but I am certain that's how it works in FBW Airbus types.

@JanHudec Cool, I actually had the misconception that the control surfaces were operated by motors all throughout

@Steve, yes. Hydraulic actuator—the pistons—are light and power can be redirected to them from one main pump for each circuit and utilising reservoirs as the peak power is never needed for long. Electric motor would be needed for each controlled element. Sometimes more than one for redundancy.

The A380 and A350 use Electrohydrostatic actuators (EHA) as a part of all primary flight control surfaces. This is a local hydraulic pump and actuator. This eliminates weight from piping and the problem where a single leak can disable the whole system. These planes only have 2 hydraulic systems as a result. The 787 has a conventional triple hydraulic system, but adds direct electromechanical drive on several control surfaces.

As mentioned already, hydraulic systems are fast and powerful but you need to also consider how extremely efficient and simple they are. Reducing complexity reduces maintenance costs which is a large factor. Flaps, trim devices, and landing gear doors are the only good places to use motors.
I believe the specific example you're looking for was the Sioux City DC 10 accident in the 80's (https://en.wikipedia.org/wiki/United_Airlines_Flight_232). The DC10 had an achilles heel in that all of the hydraulic lines ran parallel to each other in one section of the tail. When the tail engine suffered a catastrophic failure a fan blade struck through all 3 (?) hydraulic lines which led to a complete hydraulic system loss. Airplanes since then have been designed with the hydraulic lines running in separate places and further apart (i.e. different sides of the rudder) to prevent total flight control system failure.

Weight-to-power ratio of the actuators, but things are changing rapidly.
The main advantage of hydraulic vs. electrical actuation of flight controls is the easy way of distributing power of hydraulics. One large pump can be mounted onto a motor or engine, and generate power for multiple small, light hydraulic actuators.

Hydraulics do have a disadvantage that a damage to the hydraulic pipes can cause loss of control.

True - however, this is more of a problem associated with the central/distributed architecture. In existing aeroplanes, electrical is also generated by generators connected to the engine, then distributed via 400Hz power lines.

What are the issues with using electric motors? If used what are the safety concerns? In terms of safety, how different are they from hydraulics?

Electric actuators are still heavier than hydraulic ones: they require magnetic fields and metal in order to convert electrical into mechanical power, while the hydraulic actuator only requires a surface are for the oil to push onto. But the power-to-weight ratios of electrical actuators are rapidly increasing.
The safety concerns: there are fewer for electrical actuation, since it is fail passive. A hydraulic actuator can fail hard-over if the servo valve gets stuck, driving the actuator to one of its extremes.

Note that simulator motion used to be done with hydraulic actuators. The hydraulic actuators would have an audible hiss, and maintenance requirements for preventing oil leaks. The hydraulic fluid in these systems was petroleum based, that for aircraft is synthetic which is not flammable but very corrosive requiring specific anti-measures in aircraft.
Every engineer and technician is know who has worked with both hydraulic and electrical actuation has been jubilant once the transition hydraulic => electric occurred, way easier to handle, tune and maintain.

Planes use hydraulics because of the immense pressures on the control surfaces during flight. Hydraulic systems can deal with higher loads than motors of similar size. An electric motor can get "stuck" when it encounters a load that is greater than it can move, possibly causing a crash if it happened in an airplane.

A hydraulic actuator will get "stuck" just the same when it encounters a load that is greater than it can move. That is a matter of using suitably powerful actuators rather than choice of their type.

The point about higher force for size is also questionable, since the hydraulic lines and fluid add a lot of weight.

Electromechanical (EM) actuators can indeed replace conventional hydraulic actuators. There is also a intermediate, the electrohydraulic actutator (EHA) which consists of a local electric pump and piston actuator. Control is accomplished by servoing the motor rather than a hydraulic servovalve. EHAs used as part of all primary surfaces on the Airbus A380 and A350.
The Boeing 787 uses EM actuators for stabilizer trim and midboard spoilers.
In an Airbus paper on the electrohydraulic architecture of the A380, they point out that their main concerns with electromechanical actuators are with the mechanical transmission between the motor and the surface.
In particular, they note that EM actuators require gear drives which are difficult to assess for the risk of jamming and failure. They also are concerned that when the gear train wears out, it will have slack, which may lead to limit cycle oscillations. Finally, they say that with EHAs, they can be made to operate as a damper for standby and failure mode operations, which makes it easier to integrate with existing hydraulic systems.

Electro-Mechanical Actuators are capable of performing as primary flight control actuation. BLDC power density and modern controller power electronics may be well suited for the role, but hydraulic actuation has a significant head start in heritage. I'm aware of an experiment a couple decades back that replaced hydraulic actuators on half of an F-18 to evaluate just this question:
https://www.nasa.gov/centers/dryden/pdf/88699main_H-2425.pdf
As aircraft move toward "More Electric" architectures, there may be a greater drive toward EMAs throughout to minimize the required plumbing, maintenance, and system weight of hydraulics or EHAs. That said, airplane builders are typically pretty conservative...

Not sure how this answers the stated question.

Sean, I find your answer interesting and covering an aspect that was missing in other answers. Problem is that in its present form, it hardly answers the question. It's more a comment. To make it fit as a self-standing answer, you could summarize why EMA were not used in the past, and elaborate a bit on the heritage. Then you could continue with the trend to and future importance of EMA. Just my 5 cents, of course. ;)


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Is the old paint removed before applying a new one?
I've heard that paint on large airplanes weigh 250+ kg.
When an aircraft gets a new owner, do they remove the paint that was previously on or simply paint over it?
If it is removed, how is it removed? What is the correct procedure for painting/repainting an aircraft?

Yes, paint imposes a significant weight penalty on larger aircraft. On commercial airliners, that weight translates to a reduction in cargo, passenger and fuel carrying capacity, and that means less money to be made on every flight. That is why some airlines have chosen to remove most of the paint and opt for the bare-metal look. However, there are tradeoffs to be considered, such as the fact that bare metal is much more susceptible to corrosion.
These days removing paint from an aircraft is a relatively straightforward proposition if you know what you are doing. There are a number of environmentally-friendly paint stripping chemicals available that when applied to the aircraft surface, cause the paint to bubble and lift off the underlying airframe.
The paint can then be scraped off with plastic tools or removed with a pressure washer, and the stripping chemical is washed off. Other products are then used to clean remaining residue, treat the metal for corrosion protection, prime and paint.
This explanation is intentionally simplistic, but it covers the basics to answer your question. If you want to see the process in action, you might check the many YouTube videos that show time-lapse recordings of aircraft being painted. Personally. I think they are fun to watch. :)

If you're referring to airliners, the paint indeed has a significant effect on the aircraft weight and more importantly center of gravity. Airliners are mostly sold on plain white, with all but mandatory markings removed.
When a new paint-over is done, the aircraft must be weighed to determine the new weight and CoG. Small partial paintings can be made without the need to weigh the aircraft, but unfortunately I don't know what are the limitations.
Aircraft must also be weighed regularly every 3-5 years, even if no painting or other modifications are made.

Depending on jurisdiction, your comment on re-weighing intervals is probably correct for air carriers, but probably does not apply to private aircraft.

Most likely. Since the question doesn't target any specific aircraft type, category or authority, I tried to give an answer that points out the common practise. Please add or edit if you have other knowledge.

@Sami I've updated my question to include How is the paint removed?. It would be nice if I could get some answer for this with some reliable references.

@NeilMartin Unfortunately that's beyond me. Basicly, the method is the same as removing paint from anywhere, either chemical or mechanical. Fast googling resulted a few chemicals and videos using pressure washers etc.

Whether or not a repainting of an aircraft requires a new W&B is up to interpretation except in the most obvious cases, but personally I would not have any aircraft I own or manage repainted without a new W&B and flight control surface moments recalculated and logged before release to service. Anything else, IMO, falls into the "fly by night" category.

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Are there non-economic barriers to a 1000 passenger aircraft?
I know the number 1000 is just an arbitrary number that happens to have some zeros in it, but is there any non-economic obstacle to making a 1000 passenger aircraft?
Also, assume normal-ish space-per-passenger constraints, so 1087 people on a 747 isn't really a 1000-passenger aircraft.

Related: Has Airbus planned to build a larger version of the A380?

mostly structural, to build such a large craft takes some doing

The question fooot linked pretty much answers this: No and Airbus has already designed such an aircraft. That pesky 'economic' part is why it doesn't (and likely won't) exist, though. Demand for aircraft that large is very soft. Even existing A380s (and 747s) aren't getting many sales, with nearly all airlines opting for the twins instead.

Yes, there are some barriers:

Scaling laws tell us that the structural mass fraction of such a big airplane would increase, reducing its efficiency.
To evacuate 1000 passengers within 90 seconds from half of the available emergency exists will require clever engineering.

First the scaling laws: When you increase the size of an aircraft, its area will grow with the square of the increase while its volume will grow with the cube of the increase. This demands proportionally bigger cross sections of all load-carrying members. For the same reason ants cannot be scaled up and elephants have much bigger legs than antelopes. The A380 is already pushing the envelope, and any bigger aircraft will be even more problematic to design. But not impossible.
The emergency slides on the A380 were one of the most challenging parts to design. While normal slides can be mercifully short, lowering a passenger by 8 meters from the upper floor such that he or she will reach the ground at a survivable speed is hard. The slide must be much longer and still is not allowed to buckle. This required the development of new materials.
Now that this has been done, a stretched fuselage with another two or four type I exit doors will make the 1000 pax version possible. But when a future design plans to have a wider fuselage with more seats in a row, this will be one of the first limits to adding seats.

"The A380 is already pushing the envelope" - it's pushing an 80-meter aircraft stand envelope, but from an engineering perspective, I think there's quite some margin still. The wingspan is not yet limited by structural considerations. The use of composites in modern aircraft is still quite conservative (in terms of safety factor), so once these materials prove themselves over time, I should expect the A380 will become much like the 747 - massive and durable, but no engineering marvel (of course, the rest of the answer is perfectly valid, and my comment may be somewhat speculative)

If money is no object, and we ignore terminal constraints, then no, there isn't. The A380 already supports 850 passengers in a single-class layout, and Airbus has plans to make a stretched version (the wings are designed to give room to stretch the fuselage without needing too much redesign). 1000 passengers isn't really that much of a stretch; we can basically get there now. The issue is that economic obstacles are very real obstacles to building planes; no one has a need for a 1000-person plane on any route (the A380 isn't even used today with max capacity, it's used to give lots of space to first-class passengers with a bit over 500 passengers).
There are certainly at least two airfields out there that would be able to handle a 1000-passenger A380; unfortunately, neither is a commercial airport (the two are Edwards AFB and the Shuttle Landing Facility; the SLF has a 15,000 foot runway that's 300 feet wide, while Edwards has a similarly-sized paved runway). However, if operation is uneconomical, you won't see anyone building a plane to do it.

If we ignore things like evacuation regulations we can make it a single-aisle cabin, pull out the galleys and lavatories, get the seats from the same supplier as Ryanair and have a 1000-seater today.

I would assume that any runway capable of landing an A380 would probably be large enough to handle a 1000 passenger variant.  I suspect the real test are taxiways and terminals.

@SHAF I normally would too, but I know the A380 is at the very limit of the capacity of some of its runways; on the other hand, I'm sure it could fit in either of those two runways, and two runways provide proof that it's possible so I don't need more :P

@paul, please don't give Ryanair ideas... I'm already expecting a 700-seater 737 any day now.

@Jon: Don't worry. Ryanair flies from GA airfields in the middle of nowhere; no way those can handle a 700 seater.

@MSalters - you'd be amazed what Ryanair can do to save £8 per flight

Aside from finding a route that would make such a large aircraft profitable, the biggest barrier is probably that the existing infrastructure couldn't support such an aircraft.  Terminals would have to be enlarged to handle the enormous passage of 1,000 riders.  I'd guess that most taxiways probably aren't large enough either.  Even the A380 has run into trouble finding suitable airports to operate out of.

I think those are ultimately economic matters - they're not impossible or even technically difficult, they just require sufficient funding.

@AndrewMedico I think you've understated the problem though, because at some point the amount of money required to retrofit the infrastructure becomes so cost prohibitive that its impossibile.  While its theoretically possible to throw infinity money at a problem, its not probable, and infrastructure, not money, becomes the prohibitive factor.

It's not so much the terminals that are the issue, but rather parking stands. When the 747 launched, a standard 80 metre box was defined, which is still used for large aircraft stands to this day. Airbus chose to launch the A380-800 first, as it still fits in this box. Any stretched -900 version would require special stands to accommodate it - something that will be difficult to provide at some airports.

"We'd need to make airports a bit bigger" is a purely economic issue.

NASA sponsored a program to answer just this question. It was called the Ultra Efficient Engine Technology project, or UEET. If you Google it I believe you will find the best estimates for max aircraft size, around the turn of the century when this project concluded, was 2.5 million pounds MTOW. That is a bit more than 2X the size of the A380, and with current tech, that number is probably around 3 million pounds MTOW.

Welcome to aviation.SE. sources would help to increase the quality of your answer. Also, please report here the data and say how it answer the question, saying "google it" is not really helpful.

Also, saying that we can Google it to confirm your memory is the wrong way around. You should be doing the Googling to make sure that what you're claiming is actually correct.

As aircraft get larger, it becomes more difficult to make them strong enough. For example, a two-story building does not move much, but the top of a skyscraper will sway back and forth by about a yard in the wind. Aircraft have the same problem. With super materials, like carbon fiber, it would probably be possible to build an aircraft big enough to hold 1000 passengers, but would require a range of new designs and innovations, so there is a "barrier" in the sense that the builder would have to come up with a whole new set of innovative designs. All of these new designs would then have to be tested, which would be laborious. You may remember the tedious testing process for the 787 "dreamliner" that went on for years, even though it was a relatively standard design. A giant aircraft would have a testing regime that would make the 787's look like a piece of cake.
Airbus has announced that they have a potential design for a stretch 380 that could accommodate 1000 passengers.
From a practicality point of view, 1000-passenger planes would not work without changing the gate arrangements at the airport because loading and unloading would take too long. You would need to have 3 gates, 3 doors and 3 ramps all in operation simultaneously to load and unload the aircraft in a time-efficient manner. Since airport terminals are designed to have only one gate per aircraft, you would have to redesign all the airports in the world to accommodate a gigantic aircraft with a triple gate. Starting to see the problems?
Another problem is that runways might not be long enough. The larger the aircraft, the longer the runway must be. A 747-400 has a rejected takeoff distance of around 12000 feet if there is a reverser failure. Usually 747s will not operate on any runway shorter than 7000 feet and that is pushing it. If they have an RTO on a 7000-foot runway, it could result in multiple fatalities. With a giant aircraft, you might need a 12000 foot runway or something like that at least, so it would be difficult to operate the aircraft.

I'm not sure I'd call the 787 a "relatively standard design"... from the outside, yes. But on the inside it involved a lot of new applications for carbon fiber, and a switch from pneumatic and hydraulic to electric for many systems. And that's what a lot of testing was about.

@fooot Its a lot more standard than an aircraft that would carry 1000 passengers.

Other answers point out that the A380 could be stretched to fit 1000 passengers, this doesn't seem to fit with your assertion that it would require a whole new set of designs.

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Helicopter blades flapping with the cyclic – can you help with the explanation?
I am now reading books about helicopter aerodynamics. And I have one question about blades flapping.
If you push forward on the cyclic, the swashplate will tilt forward and will set the maximum pitch angle on the retreating blade, and this blade will flap up while going over the tail of the helicopter, and this is the reason for the rotor tilt forward in forward flight.
But in the dissymmetry of lift explanation, the advancing blade is making more lift, so the advancing blade will flap up while going over the nose, and that is opposite of the previous flapping movement, and causing the rotor to tilt backward. So what is the true path of rotor blade in forward flight?

So what is the true path of rotor blade in forward flight?

Simply the sum of the two movements that you described.

The rotor must be tilted forward so that its thrust possess an horizontal component which pushes it forward; the more the cyclic is pushed forward, the more the rotor tilts forward;
anyway, due to asymmetry of the lift, the rotor tends to tilt backward, partially counteracting the previous effect; the faster the helicopter flights, the more the rotor tilts backward.

So if you push the cyclic forward from hovering:
the rotor plane tilts forward $\rightarrow$ thrust tilts as well forward generating a forward thrust $\rightarrow$ helicopter accelerates forward $\rightarrow$ due to the asymmetry of the lift and the presence of the flapping hinge, the rotor begins to tilt backward partially compensating for the forward tilt due to the cyclic input $\rightarrow$ acceleration decreases until the equilibrium of 1. and 2. is reached $\rightarrow$ acceleration has decreased to zero $\rightarrow$ helicopter keeps on moving forward at a constant speed.
Obviously if you want to accelerate further forward the effect 1. must overcome the 2. Anyway at a certain point you can no more push the cyclic forward: you've reached the top speed of the helicopter.

Short typo correction:

If you push forward cyclic, the swashplate will tilt forward

if you push the cyclic forward the swashplate will tilt to the right (considering a helicopter with a main rotor rotating counterclockwise when seen from above)

Thank you for this nice explenation.                                                                                So you want to say that initally main rotor tilts in the directon of flight. and blade flaps up over the tail, but while accelerating, advancing side is making more lift, and blade flaping movement change, but maintain that tilted position based on cyclic displacement ? Are you sure for about swashplate tilting, because of the advance angle on pitch link ?

Actually if you push cyclic forward, the swashplate will tilt forward. The lift decreases on the right, so the blades reach the lowest point forward, but that's because the linkages ride the swashplate ahead of the blades. This is necessary for introducing correct negative feedback in the tilting motion—the linkages are rigged so the rotor aligns itself with the swashplate, any other configuration would result in it wobbling around.

@CiricAleksandar: yep, you have understood correctly. Obviously "initially main rotor" is made "tilt in the direction of flight" via the longitudinal cycling input.

So if I u understood right. Pushing cyclic forward cause blade to flap up over the tail because of the increasing lift, and this also produce new plane of rotation based on new swashplate position. And then after change of flaping montion, plane of rotation is tilted forward because of the swashplate position, and flapping is like it is discribed in the dissimetry of lift, and also tilted a bit to the retreating side to compensate inflow role.

@sophit The flapping movement lags 90° (well, in teetering rotors it's 90°, in rigid and articulated it is a bit less), which is why the linkages ride the swashplate 90° ahead of the blades they control. But the swashplate must tilt in the direction of the intended rotor tilt. Because that way when the rotor aligns itself with the swashplate, the lift will equalize again and the rotor will stop tilting. If the swashplate tilted 90° offset from the intended tilt, then the rotor would continue to tilt until it hit the fuselage or the limits of the mounts and broke them.

@sophit Downvoted for insisting on factually incorrect claim.

For reference https://www.spinningwing.com/the-helicopter/helicopter-swashplate/: “There is typically a 90-degree azimuth offset between the pitch link attachment point on the swashplate and the blade it’s connected to. The vertical position of the swashplate at $\psi = 90°$ feathers the blade at $\psi = 0°$.” and later “When a pilot pushes the longitudinal cyclic forward some control rods shorten while others lengthen. These motions are rigged to tilt the swashplate front edge down / aft edge up.”

@JanHudec: This is a nice picture of the swashplate assembly of the bell 222 (source Wikipedia). The blue arrow points the bar controlling the lateral cyclic, while the green arrow points the bar controlling the longitudinal. As it can clearly be seen, when the lateral (blue arrow) is applied, the swashplate definitely does not tilt laterally as claimed by you but with some 45° inclination. The same happens with the longitudinal (green arrow). The forty-something ° is to take into account the cinematics needed to get to the pitch horn...

... but it's not important for our discussion. The important think is that when a forward cyclic is applied then the swashplate does not tilt forward rather with a tilt that takes into account both the 90° delay of the flapping and the mechanical constraints of the cinematics. Hope to have convinced you now... and don't forget to retract your downvote  (btw, the link in your comment contains some inaccuracies).

Why this angle is sometimes 90 degrees, and sometimes less ?

@JanHudec: Here another example (source). This time it's a BO105 with the picture taken just in front of it. Again in blue the rod controlling the lateral and in green the rod controlling the longitudinal. And again you see that the lateral (blue) rod does not make the swashplate tilt laterally but actually more forwardly (minus the cinematics to get to the pitch horn). The same for the longitudinal (green) rod that makes the swashplate tilt more laterally than forwardly.

@CiricAleksandar: That really takes up more than a comment to answer, I'd suggest you to open a new question if you want.

@JanHudec: btw, are we clear now with this topic or not yet? Can you possibly remove the donwvote?

No, we are not. Here is a video where you can clearly see the scissor links (that's a real rotor head of a Bell-206) ride the swashplate 90° ahead of the blades they control: https://www.youtube.com/watch?v=r45SPi98rcg Because rigging the rotor so that it align itself with the swashplate is the only stable configuration, especially since it is a free-teetering rotor that is tilted by the aerodynamic forces only.

The scissor's position has nothing to do with the rigging, it's just located where it's structurally more sound. Seriously, get on an helicopter and move the collective around to see the 90° delay with your own eyes, I did it yesterday

The dominating force on the blade is centrifugal, by huge margin, so it really always moves in a plane, with the changes in lift just tilting that plane relatively slowly to the rotation.
The reason for flapping hinges or flex element or teetering is to allow the fuselage to follow the rotor tilt at its own pace without overstressing the mount.
Now both effects you mention are true at the same time, and they cancel each other out. As you push the cyclic forward, lift is increased on the retreating side, which tilts the rotor forward (with the fuselage following) and the helicopter accelerates. That increases the lift on the advancing side, and that in turn stops the forward pitching motion.
This is good for controllability, because it means the helicopter will settle on some forward speed corresponding to the displacement of the cyclic control rather than keep pitching forward.

The ‘flapping’ term is perhaps a bit misleading. It's better to view the rotor as the primary component, a gyroscope that maintains its orientation like any other free gyroscope due to conservation of angular momentum, and any changes in lift, whether caused by cyclic input or by speed differential between the sides, cause it to precess.
The fuselage is just hanging underneath. In some helicopters there is some torque carried by the hub, but in teetering rotor helicopters, and that's common design in light helicopters, it's literally just hanging there like a pendulum.
Now the swashplate is designed so that when the rotor is not in line with it, the blade pitch is manipulated so that the rotor tries to re-align itself with the swashplate. This design ensures that the rotor won't continue to precess until it hits the helicopter even when there is no torque transferred by the hub.
Except you have to add the effect of differential lift, and that means during forward flight, the rotor will actually spin tilted somewhat aft relative to the swashplate, with the difference increasing with forward speed.

So you want to say that initally main rotor tilts in the directon of flight. and blade flaps up over the tail, but while accelerating, advancing side is making more lift, and blade flaping movement change, but maintain that tilted position based on cyclic displacement ?

@CiricAleksandar well, yes. When the lift changes due to cyclic and speed difference cancel out, the rotor will maintain its orientation due to conservation of angular momentum like any other gyroscope.

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What could cause an engine shutdown on a Q400 with CDS codes 732, 907, 915 and 938?
A Bombardier Q400 displays error code 938 on the Central Diagnostics System(CDS), which is attributed to turbomachinery chip detector. An examination shows debris in the detector hitch which is collected and sent for further analysis.
Meanwhile, while on a flight the crew experiences an automatic engine shutdown with the following codes and their corresponding explanations showing on the CDS:

732-NH Speed Sensing (Connectors Eng. controls Harness  NH sensor A EEC (Freq In))
907- Engine Flameout
915-Replace Main Oil Filter
938-Turbo-machinery Chip Detected

Has anyone experienced similar problems or have any lead on the same?

Sounds like there was debris found in the engine oil. That's the typical case that a "chip detector" is looking for, and would cause the need to replace the oil filter. How that relates (cause vs effect) to the first two items, would require somebody with more experience on that aircraft than I have.

This sounds like a service request to Bombardier might be a good approach...


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Can an online course replace "ground school"?
I just found an ad for a "Learn to Fly Course."  Can this course be used INSTEAD OF taking "ground school" at a flight school?  On their site it says:

Get Your Written Test Endorsement   "... We’ll even give you the
  flight instructor endorsement to take the written test—right from the
  course! You can take this endorsement to the FAA testing center, and
  log it in your ForeFlight logbook."

This seems odd.  The flight school I'm looking at charges $1,000 for a few MONTHS of "ground school" (meeting once per week).  This online course is 25% of that cost and I can study with it 24/7.
What am I missing here?
Note:
This question was reported as a duplicate.  Someone previously asked IF ground school is necessary.  That is NOT what I'm asking here.  What I am asking here is for a COMPARISON between ONLINE ground school versus IN PERSON ground school.  I would like to know if there is some intrinsic value of doing ground school in person versus online or, in other words, if one can adequately replace the other.

There are two different questions: 1/Is is possible to learn online, 2/Why is the online education price only 25% of the ground school price. For the second question, an element is missing: Do you learn with a remote instructor (who will be paid) or is it self-education with a software or web content?

Self education is completely fine. It can save you a lot of money, and you can learn at your own pace before committing to getting a license. Refer to Dave's answer about ground school requirement in part 61 vs 141. And if you hit any questions while studying, you are welcome to ask here!

There many who learns better alone than in groups. At his/her own pace, with audio/video materials.

Which country are you asking about? We already have an answer for the US and for Canada.

Here in the states this depends on if you are training at a part 61 school or part 141 school
Strictly speaking there is no ground school requirement for a part 61 school and you are even allowed to study the material on your own. However your instructor must endorse you for the written test so many schools prefer to know that you did at least some ground school usually from them. Online is more than acceptable in this case (especially if they provide you with the endorsement) however its best to talk to your instructor before doing so and make sure they are ok with it.

§61.103   Eligibility requirements: General.
(1) Conducted the training or reviewed the person's home study on the
  aeronautical knowledge areas listed in §61.105(b) of this part that
  apply to the aircraft rating sought; and
(2) Certified that the person is prepared for the required knowledge
  test.
(e) Pass the required knowledge test on the aeronautical knowledge
  areas listed in §61.105(b) of this part.

For part 141 it depends what they got approved in their curriculum. You can find the full regulation here.

§141.55   Training course: Contents.

In the 141 case you most likely can not complete the training on line due to the fact it wont fall under their approval.
Also of note from this FAA doc

Note: If you are enrolled in a part 141 school with test examining
  authority, the school will administer the knowledge test during the
  curriculum.

Its important to note that it does not matter if you are training at a part 61 or part 141 school you end up with the same PPL.

You can find 100% of the information you need to pass the written test for free in the FAA's published handbooks particularly this one.

On a bit of a personal note, I started at a 141 school that shut down about half way through and finished my PPL at a part 61 school (with the same instructor). At the 141 school there was a rigid curriculum and almost always a half hour to hour of ground instruction prior to our flight as dictated by their approved curriculum. At the 61 school I was allowed to work the material at home before hand and come in mainly for my flying but 15 min of discussion usually occurred prior to my flight anyway. I did at least 50% of the ground material on my own and passed the written without any issues but its important to understand what you are undertaking if you go the self study route.

My wife & I both just studied at home and learned how to answer each question in the Knowledge Exam (the ones you just have to memorize are the worst) (back in ate 1994 for me, sometime in 1995 for her) and knew where the question came from.  Being college grads helped, we knew how to study.
Anything we couldn't figure out/didn't understand, we played "stump the instructor" with as part of a flight lesson. we co-owned a C150 at that time.  He had no problem signing us off to take the exam after the kinds of questions we asked. Took us both nearly a year of lessons, flying out of a little 1682 foot strip with obstructions on both ends.  Had to be really good at landings to get a plane in there before one could solo.
I did the same for my instrument knowledge exam, learned how to answer every question, and asked my instructor during flight lessons (or at work, he was my boss) for the ones I couldn't figure out.


text:
How does the airfoil pitching moment relate to center of pressure?
I've looked everywhere to find the answer and it's still unclear to me.
How does pitching moment relate to center of pressure? Does it relate to coefficient of lift?
I have made a carbon fiber wing (Eppler E423) and the CM of that wing is around -.22 where i'd be running it on my racecar.
Where is the center of pressure so I can put my main mount directly under?
PS: let me reword, I want to calculate where my center of pressure is along the chord.
I'd rather use CM to calculate that(if possible), but any other method aside from hand measuring it from a CP plot would be nice.

The pitching moment probably has little (if any) relevance in the case of an airfoil attached to a race car.
In particular, the pitching moment is related to the fact that with a cambered airfoil, the center of pressure will forward and backward depending on the angle of attack at which the airfoil is operating.
In the case of a race car, the angle of attack won't normally vary much, so the pitching coefficient probably isn't very relevant.
I'm not sure it's really relevant, but the aerodynamic center is normally taken as being at 25% of MAC. The Cm basically then defines a torque that's exhibited at the aerodynamic center.
One other point: I'd at least assume that on a race car, the airfoil is mounted upside down compared to an aircraft--that is, it's intended to produce pressure downward, not upward. To the (I think minimal) extent that pitching moment is relevant at all, it'll be positive (whereas it's normally negative for the main wing of an aircraft). That is to say, on an aircraft the pitching moment leads to a force pushing the nose downward, but in your case it'll be a force tending to push the nose up (and the tail down).
Okay, so based on the comment, what's really desired in this case is to compute the location of the center of pressure. To do this, we really need two things: the Cm and the CL.
For the moment, I'm going to assume a Reynolds number around 500,000 or so. For an E423, your quoted Cm of -0.22 corresponds to an Alpha (angle of attack) of around 5 degrees. With Re = 500K and alpha=5, it looks like the CL is about 1.6.
The formula we need is:
hcp = hac - Cmac / CL.
Plugging in the numbers, we get: 0.25 - (-0.22)/1.6.
Running that through the calculator, we get 0.3875. So your center of pressure should be approximately 38.75% of MAC. Of course, you'll need to adjust if you're actually basing your number for Cm on a different combination of angle of attack and Reynolds number.
As noted above, in your case the airfoil is (presumably) upside-down, so that should really be a Cm of +0.22 and a lift of -1.6, but the signs cancel so we get the same result either way (exactly as you'd expect).
References:

Airfoil data
Computation

Yeah, I'm not at all concerned with pitching moment on its own. I want to use it to calculate the center of pressure, because structurally, if my primary mount is below it, it would create less of a torque on my secondary mount, so I can make the secondary mount smaller and lighter.


text:
Is there a way to calculate the difference in pitch required to reach a certain vertical speed?
I am programming an autopilot for a 787 flight sim software and I've been having a problem.
I'm trying to simulate a PID that controls the pitch of the aircraft In relation to the vertical speed selected. I'm not sure how to obtain the difference in pitch required to get to a certain vertical speed in fpm and keep it stable there. Any advice and input is appreciated.
Edit:
Posted updated question on a more suitable stackexchange forum:
https://robotics.stackexchange.com/questions/16249/needed-help-with-pid-for-an-airplane-autopilot-system

Is there any particular reason why you can't just iteratively and continually adjust in response to changing conditions? If, for example, the (simulated) aircraft hits an updraft or downdraft (which usually come in close proximity to one another...), then any precomputed value for pitch to vertical speed is very likely useless anyway.

What have you tried? From your question it's very hard to see what your understanding of aircraft dynamics and control design is. I would say that the thing that relates vertical speed to pitch is your (PID) controller.

Yeah, I'm a little confused by your question. Do you know how a PID controller actually works? Because a PID controller is the thing that tells you what pitch is required to attain and maintain a desired vertical speed. If you want, take a look at the flight sim pilot that I wrote, and see how it works: https://github.com/tswett/WarrigalsAutopilot Look in the Autopilot and Controller classes.

By the way, this question would probably be a better fit on robotics.stackexchange.com, because the information you need is how to implement a PID controller. The Robotics site is the perfect place to get that information. The Aviation site is not such a good place because you don't actually need any aviation-related information here. If you'd like to move your question, click the "flag" link on your question and say that you'd like it to be moved to the Robotics site.

This is primarily a computer programming or mathematics problem.

The problem in my case is that, I have to forward controls to the yoke which moves the plane as in the real boeing plane and it isn't as responsive.

Okay, so do you need help implementing the PID controller, or have you already implemented the PID controller and now you're having problems with it? If you need help implementing the PID controller, then we need to know what, exactly, you need help with; and if you've already implemented it and now you're having problems, then we need to know what, exactly, the problems are.

I've updated the post with a link to the question i've asked on robotics stack exchange since it seems to be more suited over there.

I'm surprised no-one has attempted to answer this question, as it is mostly a pilot's daily job to guess a pitch change that would give a certain rate of climb. :-)
This will not be a technical answer, but I suspect some of my attempts at controlling the rate can be ported to a PID algorithm.
Keeping math aside, if you draw the lift-drag-thrust-weight diagram, there is probably a tan(x) dependency there between pitch and rate. For the small angles of pitch we are talking here, a linear dependency is good enough on the assumption of tan(x) ~ x . So basically, if I want to half my climb rate, I start by reducing my pitch to half and then I take it from there.
Secondly, the larger planes will still fly by the numbers, so I have a pretty good idea of how 2.5 deg will give me level flight at 250kts, 5 deg will give me around 1000fpm at climb thrust ,etc. I initially pitch to the memorized pitch angle and then adjust as necessary.
Short answer:
Build a table of pitch vs speed vs climb rates with climb thrust, and a table of pitch vs speed vs descent rates at idle thrust.  Assume linear dependency between pitch and rate. Adjust PID from there. Voila'!

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How much do the V-22 Osprey's wings weigh?
I am trying to ascertain the weight of the V-22's entire wing assembly: right, left and center including both nacelles. But excluding the engines if possible.

There is a method for deriving structural wing weight from geometric data in Appendix C, Synthesis of Subsonic Airplane Design by E. Torenbeek


text:
What rules exist that state what a competent authority do with a runaway drone?
In this AAIB accident investigation, published in 2021, a rather terrifying event is described in which a 3m long, 95 kg remote-control nearly-home-built drone is flown at speeds of up to 25 m/s below 100 ft by a crowd of spectators before becoming stuck in a climb at around 2000 ft/min. Its operators could not stop it -- its "kill switch" failed to operate -- and it rose up to the bottom of class A airspace at 8000 ft before the battery ran out. The drone later crashed in a field, with no fire, and no injury. It was intended as a large-scale demonstrator prior to a manned aircraft.
It was found that this occurred because of radio interference, the fact that the operators reduced the transmitter power of their radio control system due to CAA regulations, and arguably that the drone "was not designed, built or tested to any recognisable engineering or airworthiness standards". They received a paperwork-driven authorisation to fly with no physical inspection of the device -- and the photos show a lot of arduinos and loose wires held together with cable ties. Most of the report's 15 recommendations are aimed at the CAA to prevent an event like that ever occurring again -- clearly a 95 kg device crashing at around 5000 ft/min with flammable lithium batteries is very dangerous.
I find it odd that nothing else was done about this at the time. Within minutes of becoming uncontrollable, the drone was straight in the middle of the flight path near Gatwick and posed a danger to both aircraft and, if it crashed in the wrong place, people below.
My questions are therefore:

What actions would a country or otherwise competent authority (like the CAA) take in this eventuality, and prevent the aircraft from posing a risk to passenger aircraft?

I realise that it is clearly better to avoid it happening in the first place! Short of shooting it out of the sky (which presumably is an absolute last-case option) it seems a very sticky situation to be in -- no transponder, no functioning radio, and risks above and below.

Is there a standardised response in this sort of situation, if e.g. aircraft on the final approach to gatwick get a radio message essentially saying "be on the lookout for a 95 kg, 3 m long, untracked drone flying vertically upwards at 2000 ft/min somewhere nearby"?

Would ATC try to invite flights to use an alternative approach path -- e.g. even one that involved landing with a tailwind, to avoid the affected area?
If it helps, here are some pictures of the drone in question, all taken from the AAIB report:

(And a Youtube summary published by the AAIB)

I've tried again to reword this to make it less opinion based. If anyone else wants to have a go, feel free -- I hope that the question is of interest at any rate. My PPL training has unsurprisingly not covered "an unknown, uncontrollable aircraft may hit you from below" -- rather just that I should give way to uncontrollable aircraft (like balloons) if I encounter them.

Just don't forget that the sky is full of lots of things besides runway drones-- like birds--

The scariest thing for many government bureaucrats is any new area of enterprise. Those things tend to evolve quickly, and often in unexpected directions, requiring adaptation which is difficult when you like things just the way they are.
To keep things simple, I'd answer your two questions with:

Regulators can easily prevent these kinds of events by enacting expansive and expensive regulations that effectively kill all further activity in the field. The downside is that it's like killing your horse to keep it from escaping the stables.

I fly often near an area where the government tests various types of UAVs. The ATC mentions their presence at times, just like for any other traffic. Sometimes they show up on the don't-hit-me idiot box, too.

If you want my 5c, I'd say that once a UAV gets large enough it needs a transponder so others will see it easier in their Nintendos. But it's not just about size, because even a large object is pretty much harmless if it has low density.
Whatever the optimum formula may be, I expect regulators to get it wrong if they try to define it too early. The worst thing would be for them to use early outlier events to define policy that kills a promising area of air mobility.


text:
Is the math behind my approach to design a propeller right?
I'm trying to design a fixed pitch propeller of diameter 1.8 m flying at 10000 ft going at forward velocity(85 m/s).
What I've done is take a series of airfoils (from root to tip), found the respective angle of attacks for each airfoil for which the $C_l/C_d$ ratio is max. I've then Added that to the angle of my resultant velocity (which I got from using 2300 rpm*(airfoil $r$) and an 85 m/s forward speed). This is now the angle at which my airfoils sits with respect to the vertical. Now for chord lengths, using the blade element theory,
$$
\mathrm{d}T=qc(\mathrm{d}r)\frac{C_l \cos(\phi)-C_d\sin(\phi)}{\sin^2(\phi)}
$$
I've take $\mathrm{d}r=0.001\,\text{m}$ to approximate the $\mathrm{d}T/c$ for each small section of airfoil. I know $q$, $\phi$ and the co-effs. Now I have the $\mathrm{d}T/c$ for each airfoil that I've taken. Then I took the airfoil at approx more than half the overall radius and set  $c$ as 0.15 m. So now I have the $\mathrm{d}T$ for that particular airfoil. Assuming I want thrust to be constant through out the propeller I used this value of $dT$ and the respective $\mathrm{d}T/c$ for the airfoils to get the remaining chord lengths.
Using these values for angle of attack and chord lengths I modelled my propeller.While modelling I reduced the chord lengths at the roots since the roots are more for structural stability than thrust generation and having a large value as chord length at the root could compromise the propeller.
I know that my propeller is modelled for a very specific forward speed, but I want to know if the math I used to approximate the chord lengths checks out. It seems to be okay since I'm getting the resultant angles to decrease along the length of the propeller and so do the chord length. But my theoretical overall thrust is around 3000 N while my CFD model is 1000 N (efficiency=60%). So I'm not sure if my model is bad or my theoretical calculation is off to the point I can't present it and say "This is why I've used these as the chord lengths"

You're using a uniform thrust condition to set chord.
The Betz condition for a minimum induced loss propeller (best efficiency) is a uniform induced velocity.  You may want to look at reformulating towards that.  It will probably take an iterative solution (not just one pass through).


text:
Are there Cessna spare parts supplier in South East Asia?
I am living in South East Asia, and thinking of buying a reconditioned Cessna 172. Are there any spare parts supplier here that I could purchase , or do I need to order from US ?

Something like 8.5% of the world's population live in south east asia, and the Cessna 172 is the world's most produced aircraft. While the details depend on what country you're in (e.g. I suspect getting spares for a US built aircraft is harder in myanmar / burma, for example) if there's a place where you could get spares and (far more importantly) a qualified mechanic, it's very likely that they'll have come across the Cessna 172 before. It's the archetypical civil general aviation aircraft, and in the (unlikely) event that you need something specific and exotic that your local area doesn't have, spares are very common worldwide and Fedex is comparatively cheap – at least in comparison to aircraft parts!

If I were in your shoes, I would start here, at Textron Aviation’s locate service portal.  To purchase a new Cessna aircraft, you can locate a Textron/Cessna sales rep in your area here.  There appears to be a Textron service center in Singapore and they will probably be able to help you there.
Keep in mind, for many countries in that area of the world it may be difficult to find service or spare parts. Light airplanes are kind of a first world luxury item and most OEMs are not going to spend the money on maintaining facilities in parts of the world where people cannot afford them.  Countries whose governments are communist and/or have unfriendly relations with the USA present even more difficulties (if you live in the DPRK, you might be able to buy the Dear Leader-approved, and unauthorized, local built C172 clone.) You would either have to go to Singapore or Australia or some other place where major repair and overhaul facilities are available.


text:
Why circle an airport after takeoff? (SLC particularly)
I left SLC (Salt Lake City) this morning heading to MCO (Orlando). On previous flights, I remember takeoff similar to other airports. We got in the air, gained altitude, and moved to our heading and left.
This time though, we seemed to circle for a while, gaining altitude. I initially assumed it was to gain enough altitude to clear the surrounding mountain. I ruled out equipment problems because we kept climbing.
Pilots who have experience with SLC, is there a certain runway that needs this altitude gain due to the terrain? Would there be another possible reason this happened?

Did you take off to the south or the north?  There are a couple of departure procedures at KSLC that call for a hold if vectors aren't received in time...

I don't know which direction.

I found this (PDF) after @UnrecognizedFallingObject made his comment. I do see on Page L12 that SLC has some obstacle procedures.

However, none of the SLC ODPs involve circling (a climb-in-hold, in other words).  Look at IFP (Laughlin/Bullhead Int'l) or CGZ (Casa Grande Muni) in that document for examples of a departure procedure (textual obstacle SID) that has a climb-in-hold.  (There are several others as well: EKO and FLX also have this, for instance.)

you'll need the charted SIDs for this -- in particular, look at the SALT LAKE TWO and SEVYR ONE charted SIDs (you'll want to look at both the chart and the associated text page).

I would assume it was to avoid terrain, traffic and/or the restricted airspace close to the airport, but there is at least one departure procedure from KSLC that calls for a climb in the hold. The SALT LAKE TWO departure includes this:

Continue climb in TCH VORTAC holding pattern (hold south, left turn,
  341° inbound)

And several other departure procedures call for climbing right or left turns; combined with a few ATC vectors that could easily give the impression of circling.
You might try looking for the actual radar track online if you search flightaware.com or a similar site for your flight. That would let you see what track you really flew.

To avoid terrain or obstacles. Admittedly, I don't know specifically which runway I took off from, making it difficult to provide a definitive answer for this specific airport.

There is a rather large mountain range directly east of the airport.

authors: ƒиαƒƒσχу 2196


text:
Are there any aircraft that are capable (or can be modified) to be bush planes for cargo
Are there any STOL planes that are cargo carrying bush planes? (or can be modified to be bush planes for carrying cargo out of remote areas?).
I have clarified my question, based on feedback from the community, and am now no longer considering homebuilt aircraft, since they cannot be licensed to carry cargo (for now, I'm only interested in non-human cargo).
Needless to say, because of my budget, I suspect that I'm looking at previously-owned (i.e. secondhand) aircraft.
Criteria being:

manufacturer still in business, long track record of safety
relative inexpensive to buy (roughly around $100k or less if possible)
Easy to fly and maintain
Relatively low fuel consumption
STOL
Easily available, inexpensive parts (does NOT rely on unique, custom hard to find parts)

Performance requirements are:

max payload: 500 Kg
acceptable min, max range: 250 Km, 500 km

Your question is too broad – you need to specify what kind of performance (e.g. payload, range, amongst other things) you want. Remember also that most countries ban any commercial operation with aircraft from the experimental category.

@Chris Thanks for your input. I have narrowed the scope as you suggested and also removed the requirement that it be homebuilt, as those can't be used for transporting cargo.

The obvious choice is an older Cessna 185 with a STOL conversion kit that includes vortex generators, although a decent Skywagon is going to cost a lot more than 100k (200-300k). If you want to haul 1200lbs with full fuel, or close to full fuel, an older 185 is about as cheap as you're going to find that meets your requirements. Plus the manufacturer is still in business and there is a huge salvage parts market.  It's a no brainer.

Thanks this is what I needed.

If you want to go a next step up to a ton+ of payload, the DeHavilland Canada Beaver is the go-to machine. A 75 year old design that is still the preferred machine for a lot of bush operators, and great STOL performance.  2/3rds of Beaver production was for the US Army, starting a long relationship.

Not in the United States, by regulation experimental aircraft may not be used to carry passengers or property for hire.
14 CFR 91.319
If by cargo you just mean personal belongings, then many high wing aircraft can be modified to carry cargo up to the MTOW and CG limits of the aircraft.

Thanks Ron, your answer made me realise that homebuilt aircraft do not fit the bill. I have therefore, modified my question in light of this information, as well as the comment made by @Chris.


text:
FAA Turbine engine FCU regulations
Does the FAA have any regulations governing the design of a turbine engine FCU? I'm being questioned about an older Beechcraft PT6A FCU - in particular, how can I guarantee that when the power condition lever is at the shutoff/cutoff position, no fuel is moving to the combustor...? Any help would be appreciated... Thanks!
Edit - for those needing clarification:
Does the FAA have any regulations governing the design of a turbine engine FCU?
Simply as stated, does the FAA regulate how a turbine engine FCU can be designed - pressure allowances, flow volumes, etc.
I'm being questioned about an older Beechcraft PT6A FCU - in particular, how can I guarantee that when the power condition lever is at the shutoff/cutoff position, no fuel is moving to the combustor...?
As stated, I'm being asked to prove that when the power condition lever in the cockpit is moved to the shutoff/cutoff (pick one term) position, fuel will not flow into the combustor. Tying into my first question, does the FAA regulate that a turbine FCU MUST absolutely cut off all fuel moving to the combustor, or is that just left to the designer to ensure...?
Hope that clarifies everything. Not sure how much more detailed I can get without specific and possibly pointed questions.

Thanks @RalphJ, the question was answered below. But I appreciate the effort!

@JediEngineer Yes the question is answered, but it is still unclear and about to get closed. Please rephrase it so that it can be a valuable question.

@fooot.  Can I please ask why this question is on hold? It might be unclear to you what they were asking, but it was crystal clear to me. My answer has been accepted, so my understanding must be correct. So, logically, then the decision to put it on hold because "it's unclear" is wrong. Can you please remove the hold on this question. Maybe the answer was accepted after you put the question on hold (if that is actually possible).  Alternatively, can it be explained why a question that has an accepted answer, and therefore must be an appropriate and correct answer, is "unclear"? Thank you.

@Penguin The answer was accepted before it was put on hold. And fooot alone did not put it on hold, several people think that. Since the question is crystal clear to you, can you please make it clearer for those who did not understand it and then it will be submitted to be reopened.

@foot, I'm not entirely sure how much more I can clarify this question but I'll try. But it's pretty straight forward if you read it sentence by sentence.

It has been clarified.

@Farhan.  Thanks very much for your positive reply. (I realise a number of people decided the question was unclear, but could only put 1 name in my comment. I also realise no one is paid to moderate.) Thanks again.

Subpart E (Powerplant) of Part 23 (AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES) has quite a few regulations regarding fuel system operation. In particular, section 23.995, "Fuel Flow".
However, I could not see in there a regulation requiring no fuel to flow in the off position, but given the safety implications of that not occurring, I would fully expect that is a requirement that needs to be demonstrated.
It appears to exist in Part 33 (AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES), subpart E (Design and Construction; Turbine Aircraft Engines) section 33.75 "Safety Analysis". This states:

The applicant must analyze the engine, including the control system, to assess the likely consequences of all failures that can reasonably be expected to occur.... The safety analysis must also include, but not be limited to, investigation of the following: (vii) complete inability to shutdown the engine.

I don't expect there are any regulations defining how it needs to be designed to do this - but rather regulations saying what it needs to do, and the way of achieving it is left up to the designer.

thank you so much! This leaves a little to interpretation, but it has solved my issue. Thanks again!

I was able to further trace that regulation back to the year the PT6 was first created, to further find that the FAA didn't exist back then, and aircraft were under a 3 page regulation from the CAA, which grandfathers in under the FAA regulations. Turns out that the FAA doesn't say a turbine FCU must cut off the fuel when placed in that position, but it is considered a hazard if it doesn't.

@JediEngineer.  Interesting, but quite believable since the PT6 is kind of the engine version of grandfathers axe! It's been around for a long time, and possibly every part has been changed and improved, except for the name...!


text:
Ground Turning Radius
Regardless of any legal regulations, is there a standard technique for designing the ground turning radius of an aircraft, or is that typically just a fallout parameter?

Related: Is there a taxi turning radius requirement for normal category aircraft?

It's similar, yes. But my question is more about design technique vs the legal regulations.

Changed my comment accordingly ;)

It's a fallout parameter. The procedure for placing the landing gear during the pre-design are as follows:

Main gear is positioned behind the CG projection to ground when the aeroplane is in its touchdown angle.
Wheel track is determined by preventing sideways turnover.
Nose gear distance from main gear is then determined such that the load is between 8 and 15% of MTOW.

Minimum turning radius then follows from turning with the nose wheel deflected at 90°.

Some things to consider:

Don't make it a potential weak point on the aircraft. This was/is a big problem with some of the Mooneys out there where the nose gear can be damaged if a very tight turn is made while towing:

One other caution: Many Mooneys suffer damage to the nose gear trunion
  when towing turn limits are exceeded via power towing. Owners learn to
  watch the ramp rats carefully.

Make sure you can maneuver the aircraft around the desired airfields. If you are building an aircraft that is intended to operate out of small GA fields with tight taxiways and short runways, keeping turning ability tight is key. If you are designing an airliner, make sure you can get around major airports (taxi diagrams are readily available). You don't want turning radius to be a limiting factor when buying/selling an aircraft.
To caster or not to caster, there are plenty of aircraft out there with castering nose wheels steered with differential breaking only this might have an impact on radius but a benefit in simplicity of design that is for you to decide.


text:
Why would FlightAware show this meandering flight path?
Today's BA219 from LHR to DEN (2018-03-20) appeared to fly 5 zigzags just before and over the Hudson Bay in Canada at around 18:15z. Prior to that, its flight path matched its planned route very closely.
I know that congestion avoidance can sometimes lead to flights being asked to fly doglegs (as in Why did this plane fly in a zigzag pattern?), but this flight was still 3 hours from its destination and still exceeding planned speed when it appeared to fly zigzags. I don't see any weather issues on radar at the time.
After the zigzags seen in the FlightAware capture, the flight track straightened up and flew parallel to its planned route. This appears to be a track data problem, not the actual plane's path.

But on FR24 that path is not there:

I've seen 'Why do aircraft on Flight Radar 24 jump around randomly sometimes?', since they are different services, what could be the reasons for FlightAware to behave like this? Also note the linked FR24 post is a different more jagged path than the one here.
FlightAware tracks from other flights show similar zigzags in the Hudson Bay area. Presumably this is just noisy tracking data. Any idea why that happens in this particular location?

Thanks, @Sanchises. I started with that question. The answer in that one is that the zigzag was a dogleg for congestion avoidance. I mention in my question that that seems less likely here since this flight was 3 hours from landing and maintaining an above-planned speed.

@ymb1 - The zigzags I'm seeing with this flight on FlightAware look much more "believable" than the Flight Radar 24 discontinuities. These look like actual turns.

It sure does, @ymb1 ! That's even curiouser.

Since it's somewhat different (a possible FlightAware tracking problem that behaves quite differently than the FR24 issue), would it be better for you to create an "answer" that I can accept, @ymb1 ?

FlightAware tracks for other flights show similar zigzags in the Hudson Bay area. Presumably @ymb1 is right that it's a data issue and not the real flight path. I'm still curious why the data would get sketchy in that area.

Probably a rounding error/coordinates having a few digits cut off

I was also thrown off by the apparent smoothness. Perhaps FlightAware uses some kind of smoothing algorithm?

It does seem to be smoothing things. The track log shows single entries for a course 150 degrees, from Edmonton Center. Later entries from Edmonton are better but still a bit wacky.

That sounds like the issue - a few wonky location points that are then connected with an artificially smoothed track. I can't +1 on comments, but presumably stackexchange would let me accept an answer if you add that as one.

It looks as if the GPS location was only updated at intervals, and the software extrapolated the missing path information using bezier curves.

Even in that case, why would it assume it wasn't flying the original path? A Bezier can be a straight line if the control points are in a line, and in this case, they should be in a line.


text:
How much does weight really affect the operation cost?
Hi given an information from a reliable new source article (https://www.nytimes.com/2008/06/11/business/worldbusiness/11iht-air.1.13628276.html), it was mentioned that "Every 25 pounds we remove, we save $440,000 a year,". I wonder the validity of the statement because it goes against basic math.
Here is what I got,
\$440,000/yr/25 lbs = $17,600/yr/lb
Let's say a passenger weight at 200 lbs, this means by removing one passenger out of the plane, an airline would save   $\dfrac{\$3.5 million}{year \ person}$.
That means that one passenger should be paying more than that amount over the year, but if we think calculate that by using a ticket from L.A. to New York as an example, that's not the case (calculation shown below).
$ \dfrac{\$262}{person}$ $\times$  $\times$ $ \dfrac{4 flights}{day}$ $\times$ $ \dfrac{365 days}{year}$ =
$\dfrac{\$380k}{year \ person}$
I know there must be something wrong in this simplistic calculation, but I couldn't figure out. I am hoping someone here could point that out (with a reference would be awesome).
Thanks in advance.

A couple of old references behind paywalls estimate commercial aircraft weight reduction is worth $300-800/lb over the life of the aircraft, and it is heavily fuel price dependent. This was back when fuel was half the cost it is today, so double it.
Your reference quotes a Northwest Airlines manager who may have been describing savings across their whole fleet of 320 planes. This drops the annual savings to 50/lb/aircraft, or $1500/lb across the 30yr life of an aircraft, roughly equivalent to my two sources.

I see, thank you @Pilothead


text:
What is the english term for "vol en ficelle"?
From what I have read online and understood, "vol en ficelle" is basically a low altitude flight preparation method that requires calculating a set of altitudes to be followed in order not to crash.
It consists of flying near the ground, but following small portions of levels (so without trying to flight closely the contours of the relief).
It is then necessary, during the mission preparation phase, on a predefined route, to cut it into sections and determine, for each of them, an altitude (not a height) that allows to keep a sufficient margin relative to the ground.
During the flight, it is important to know with sufficient precision where you are, then fly at the preset altitude.
The flight preparation is the main part of this technique, where you have plan properly the flight course and accorrdingly define corresponding landscape and montains heights.
Below is a map of such preparation. On the left side of the map is the "ficelle altitude", this flight preparation technique is apparently also called the "méthode numéri"

Image source : Le diable a bord
EDIT:
I doubt it is contour flying, even though that's the closest term to "flight en ficelle" so far.
Contour flying means flying at very low heights by maintaining a constant height above the ground, which means the altimeter is going to vary widely depending on the terrain profile. But with flying "en ficelle", the pilot follows a predetermined set of altitudes, thus we can fly without radars and the altimeter changes only in successive levels, i.e map in the OP. In "Flying in ficelle" the bulk of the job is the planning, ficelle refering to the thread used in the mapping technique to find out the "predetermined altitudes".
EDIT 2:
In the HUD of the Mirage F1, the "ficelle altitude" that the aircraft should go not lower than, is represented by the reticle below.

Rings a bell?
Are we overthinking this ? It is basically a low flying technique with predetermined set altitudes calculated in advance.

What is this technique used for?

I got most of the info from French pilots, it was used back then to be able to flight at very low altitude, at daytime, at night even with bad weather conditions, using the surrounding montains as covers. Following the set of pre-determined altitude, the pilot could fly mainly in a straight lines, without radio transmission and without the radio altimeter. So in a sense it was to fly without being detected by enemies.

It doesn't seems to be used anymore now that better avionics are available, and specifically used by the french army so I doubt there is a direct translation

Update:
I found this translator-help page, which suggests:

[low altitude] constrained flight path

Perhaps nap-of-the-earth flying (Wiki) is what you're looking for.
According to wikipedia:

Nap-of-the-earth (abbreviated NOE) is a type of very low-altitude
  flight course used by military aircraft to avoid enemy detection and
  attack in a high-threat environment.

It also mentions other terms such as (emphasis mine)

Other, mostly older terms include "ground-hugging", "terrain masking", "flying under the radar" and "hedgehopping".

Image source

Thanks for the informative reply, I think we are getting close to the term. With that being said I do not think "vol en ficelle" corresponds to "nap of the earth" because I dont think the purpose of fyling "in ficelle" is to avoid enemy detection.
I think "contour flying" is closer to what we are looking for here.
Contour flying definition is given as "Flying at very low heights by maintaining a constant height above the ground and thus following the terrain profile. It should not be confused with nap-of-the-earth flying." Source: http://aviation_dictionary.enacademic.com/1656/contour_flying

After seeing your link, I think 'contour flying' is indeed what you're looking for.

Hmm I wish it was what I am looking for but I think unfortunately its not.
While contour flying it pretty close to "vol en ficelle", there's one major difference.
Contour flying means flying at very low heights by maintaining a constant height above the ground, which means the altimeter is going to vary widely depending on the terrain profile.
But with flying "in ficelle", the pilot follows a predetermined set of altitudes, thus we can fly without radars and the altimeter changes only in successive levels, i.e map in the OP. In "Flying in ficelle" the bulk of the job is the planning


text:
Was the safe landing of Southwest flight 1380 extraordinary or expected?
I've been reading with great interest and concern about the recent Southwest flight that resulted in the first fatality in that airline's history.
There's one question I haven't seen answered, though: Was the plane really in serious danger of crashing?
I have long understood that two-engine commercial airliners are designed to be flown safely with one engine. However, are they designed to be flown safely with only one engine as well as a depressurized cabin and broken window? Do these three things in combination make a plane significantly harder to fly or land than just an engine failure?
The passengers aboard this plane had plenty of reason to be terrified, what with oxygen masks dropping from the ceiling and a woman nearly being sucked out of the plane (and ultimately dying from the head trauma she suffered), but the accounts I've read seem to suggest that many thought crashing was the most likely outcome and were preparing for their death.
I don't want to take anything away from the pilot, Tammie Jo Shults, who clearly performed superbly under stress and exhibited the highest level of professionalism, but was her safe landing anything other than the expected outcome?

I think, all things considered, the applause should go to the crew, not necessarily for “piloting an out of control airplane,” which it wasn’t, but rather staying calm and collective in a frightening situation.

Based upon the damage done to the aircraft, the fact that it retained a functioning engine, and the location of the engine failure and divert, no there was not a real immediate danger of crashing.
To be fair this incident was an emergency which caused structural damage to the aircraft and breach of the cabin structure, resulting in the loss of life of a single passenger.  It was also a freak accident and I don’t see any evidence that the aircraft or engines used have some kind of inherent design or engineering fault that prophesies more failures or future risk.
It’s possible that we did dodge a bullet and that, by pure chance, the failure of the fan blade and where it was ejected from the fan casing could have resulted in far more damage to the aircraft and critical systems like flight controls, fuel, hydraulics, landing gear, major structural members, etc that COULD have resulted in the very realistic probability of a much more devastating accident when the aircraft either attempted to land or could not maintain level flight further.

Note that the blade seems to have been actually contained by the casing. However subsequently the inlet cowling, well ahead of the actual fan, then broke off and caused the damage. Why it did so is subject to the investigation—actually two of them, because it already happened once. Since it did not have much energy, it is unlikely it could cause much more damage than it did.

If the engines don't have an inherent fault, why is the FAA issuing AD's on them?

@fooot They tend to do that when high visibility issues arise. It’s usually out of an abundance of caution, and not some nefarious “We knew this day was coming,” kind of reaction.

@Frank I just don't think the FAA issues emergency AD's with widespread repeating inspections just for appearances.

The issuance of an AD doesn’t not mean a bad or a inherently flawed design, but the details on this won’t be fully known until the NTSB report on this matter is complete.  What we do have is an engine design with millions of flight hours logged on it with a good safety record and really nothing to indicate negligence in their design and construction.

@fooot Why not? Look at it this way: Let’s say they don’t issue an AD and it happens again. Heads would roll. The lawsuits alone would be ridiculous. Let’s say they do find a few other blades failing. Then you’ve prevented another possible event. And if you find a bunch of blades failing, then you’re a hero. One other thing of note: SWA already started inspections immediately. Sometimes the FAA issues these AD’s to force cheapskate carriers who aren’t so keen on safety into performing inspections. SWA probably had theirs done by the time the AD was issued.


text:
Can aircraft fly without wings?
Is it possible that a aircraft can fly without having wings? I am wondering if somehow it can fly then will aircraft will be able to maintain the balance.

highly related: Can an aircraft fly without wings and stabilizers?

highly related: Do airplanes need wings?

Yes. Soyuz, Shenzhou and Apollo reentry vehicles and all sorts of missiles. And balloons.

"With enough thrust, anything can fly"

Yes, they can, these are called lifting body aircraft.

Source: Wikimedia
They maintain directional stability using the control surfaces near the tail.
If you like retro documentaries, here's one on YouTube about lifting bodies done by NASA.
As for aircraft that normally have wings, they can sometimes fly when losing (most) of one of them:

thank you, very much. @ron-beyer

Dunno if the F-15 story would qualify on this one as that guy was just very lucky.

Why not? It wasn't all luck, the F-15 is basically a lifting body at higher speeds, I believe they found that the engine intakes in this case were providing a lot of lift. Either way it's just an example of aircraft flying without both wings.

Space ship reentry capsule

Patriot PAC-2 missile

Balloon

Boeing X-51, X-51A and other wave riders


text:
Why don't they build airplanes from 3D printer plastic?
I'm not an expert in this field.
My assumption is that there are plastics lighter and stronger than the materials used to build planes. So that would make plane lighter and thus would consume less fuel which pollutes less and so on.
Maybe it would be more maneuverable, go faster, be safer as you may be able to safely land it with a number of parachutes in case of engine failure for example.
If these are true, why are planes not built from such a lightweight plastic?

Comments are not for extended discussion; this conversation has been moved to chat.

While there are plenty of answers explaining it very well let me say that Additive manufacturing (ADM) is starting to pick up quite strong in the aerospace industry for different parts of airplanes. ADM is basically 3D printed metal.

My assumption is that there are plastics lighter and stronger than the materials used to build planes.

That is not a correct assumption.
Typical 3D printer plastics have a best-case tensile strength of 45-50 MPa.
Aluminum 7075, a common aerospace alloy, has a tensile strength of 500-570 MPa.
After dividing by specific gravity, that's at a 1:4-1:5 ratio of specific strength in favor of metal. There is no significant application in which 3D printer plastics, the properties of which are driven by viscosity, adhesion, and other printing qualities, would offer better strength-to-weight than aerospace metals and fiber-reinforced composites.
Some amount of 3D-printed plastics will likely appear in cabin interiors, for low-volume parts or complex hollowed-out parts, not subjected to significant stresses. But load-bearing parts require high strength and high strength-to-weight ratio. If it's too heavy, it won't take off, and if the material's too weak, it won't stay in one piece.
So the brief answer is, they don't build airplanes from 3D printer plastic (or clay, or plaster, or thatch) because they want them to fly.

Comments are not for extended discussion; this conversation has been moved to chat.

Let's take ABS, which is an extremely common plastic used for 3D Printing. At a typical flight altitude, the exterior air temperature will be in the order of -51°C/-60°F. The lowest rated temperature for ABS is -20°C. I hope you can see why this alone might be a problem.
Wikipedia also says that ABS and PLA, which is the other major 3D printing plastic, are damaged by sunlight. Planes generally see a lot of sunlight.
Also, in regards to "you may be able to safely land it with a number of parachutes in case of engine fail", two quick points:

although plastic is less dense than aluminium, it's not as if the plane is suddenly going to be that much lighter
planes are decent at gliding in the unlikely situation of all engines failing

More information about why parachutes don't make sense can be found at Why don't big commercial planes have full aircraft parachutes?.
In case you weren't aware, the Boeing 787 fuselage is principally made up not of aluminium, but of a carbon fibre reinforced polymer, which is a material based off of plastic. So there is work going on to make sure planes out of different materials, but it's not particularly simple or straightforward.

3D printed ABS is much weaker than machined or molded ABS.

@EricShain making the case for using it even weaker...

And carbon fiber parts generally have strands of fiber oriented in different directions in order to give it strength and/or flexibility.  (For a perhaps more familiar analogy, compare plywood to boards of the same thickness, which easily split along the grain.)   AFAIK, that sort of composite construction is impractical, if not actually impossible, with current 3D printing tech.

Why would you 3D print in the first place? It’s great for prototyping or small production runs, but for mass production it’s too slow and therefore expensive.

@Michael the main use cases are complex geometries where AM allows to slowly produce a monopart where conventional techniques would require an assembly, for example rocket engine nozzles

@Michael Aircraft manufacture is small-run. That's why 3D printing is commonly used in the industry for interiors, ducting, manufacturing jigs, etc. Just not for structural parts (yet).

@jamesqf: If you think just a little bit outside todays 3D printers, laying fibres into 3D printed base material would be piece of cake (with 3D feeder). In fact, that's how it is layed in real composites. But still, you're making a correct point. It's still not suitable for critical parts. Maybe in the future some special holders, handles, brackets, etc. may be produced that way. Speaking of interior parts, some of them in real airplanes produced in past were (IMHO) of poor quality anyway.

@myself: Well to be exact, laying particular fibres is just one of the methods and more common is laying whole fabric layers. But I think the point is clear.

There's a few things going on here.
You assume plastics are stronger and lighter than the metals and composites used for aircraft manufacture today. Therac's answer counters this assumption (spoiler: plastics are much weaker and less stiff), but there's a subtlety. In many applications, 3D printed plastics can be made stronger or stiffer for the same weight, because complex shapes (that can't be machined from metal) can be printed hollow. It's the same kind of advantage you get by replacing a solid beam with an I-beam or truss, but workable for smaller scales and more complex geometries. But the structural parts of an aircraft are typically the skin, which is made of thin sheets, and the ribs and other members, which are already made with trusses, I-beams, and other geometry optimizations, so it's harder to make up that difference.
Even though you wouldn't want to build the airframe or engines from plastic (whether 3D printed or manufactured by another technique), there are a lot of plastic parts in today's airliners, and many of these are 3D printed because it's cheaper for the size of production run than creating the tooling necessary to injection-mould them. 3D printing is also increasingly used for spares or modifications for end-of-life aircraft types where original parts are scarce or expensive. Longer-term, there's the potential to drastically reduce the need to distribute spare parts to all the places your aircraft are. Right now, if you run an airline and you fly into some regional airport, you have to trade off the risk of an aircraft being stranded there lacking some spare part vs the cost of keeping an inventory of spares on that site. This isn't just flight-critical spares: say all the toilet seats on your aircraft break, and you don't have any spares on-site, you might need to cancel flights and fly your aircraft empty to a "hub" where you have the spares, which is very costly.
Getting 3D-printed parts (or any other method of manufacture) certified is a long process, because they need to be safe, reproducible, and traceable. The printer needs to guarantee that every part is within tolerance, and each part needs to be traceable back to the original plastic shipment it was made from - so that the correct aircraft can be grounded, in the unlikely event that a bad batch makes it through inspection. Only certain materials can be used for interiors, because they need to be tested to ensure they don't release toxic smoke if there's a fire. This testing isn't to the level of "PEEK is fine", but "this particular brand of PEEK filament, made via this particular process in this factory, is fine".
Despite this long path to certification, the two biggest aircraft OEMs are shipping aircraft today with hundreds of 3D-printed components.
Although you specifically mention 3D printing with plastics, 3D printing with metals is a growing field. Some boutique automotive companies are using 3D-printed titanium engine parts, because they can print structures that aren't achievable with machining, to reduce weight. Although these techniques are less mature - getting the dimensions right is still a challenge - metal printing of non-structural aircraft parts is already starting to happen.

"you'll be flying with metal, 3D-printed aircraft parts within the next decade." – According to a link posted in a comment to the question, a mounting bracket in the A350XWB pylon is 3D-printed titanium. It's not exactly the main wing spar, but pretty darn close to "structural", I would say.

@JörgWMittag Major structural components aren’t that far off either.

You state: "in today's airliners, and many of these are 3D printed...". Care to provide a citation for that, because I kind of doubt it.

I don't know whether metal 3D-printed flight-critical hardware is in use in aviation, but if nothing else the Electron rocket uses the Rutherford engine, which is mostly 3D printed.

@EricShain "Since 2015, Airbus has installed thousands of FDM flying parts on aircrafts.", New Airbus A350 XWB Aircraft Contains Over 1,000 3D Printed Parts, "More than 60,000 3D-printed parts are already flying across Boeing’s commercial, space and defense product ranges"

@DanHulme Best to incorporate the links in the answer, since comments may be deleted at any time by the moderators.

Laser Sintering is a method that is fully mature and has been used in aeronautical mechanics. It's main application is ceramics and metals, and there have been research on it

As other answers mention, the strength of molded or printed plastics is an order of magnitude lower than that of typical aerospace metals. But also the stiffness is much lower. Compare Al 7075, which Therac's answer mentions, with Stratasys' certified Ultem 9085 thermoplastic. The aluminium has an elastic modulus around 70 GPa, while the Ultem's elastic modulus is 2–2.6 GPa (depending on how it's printed).
We once tried to build a wind tunnel model in a 3D-printer. Looked nice when it was finished. But when it was subjected to the loads in the tunnel, it warped out of shape horribly. The results were unusable. A 3D-printed wing constructed like a conventional wing today would not only break, but before doing so would warp and twist completely out of shape.
Another weakness is UV sensitivity. While metals can withstand years of intense solar radiation unharmed, the bonds in polymers suffer form the high energy of UV rays (PVC is an exception but has got an undeserved poor reputation), so any 3D-printed surface will decay in the open. Protective coatings are only of temporary help and add weight.

Was the structure ABS as well? I've had decent results using 3D printed ribs and panels over a composite structure.

@AEhere: I think your composites provided the stiffness. The 3D model was from UV curing resin. If you restrict the printed parts to ribs and secondary structure, 3D printing is no problem.

Well, yeah, I specifically disregarded the aeroshell since it was an unknown quantity with the printing quality I had and sized the spar to match. I ran the numbers on an ABS spar and it would have been massive to hold up. Haven't worked with UV resin but from what i have read it is not too far removed from ABS or PLA. I guess my question should have been if the structure was not strong enough or if stuff like the leading edge bucked in due to dynamic pressure.

The plastics used for airplanes are a lot like reinforced concrete, where both high compression and tensile strength is needed.  The plastic compound itself, polysester, vinylester, or most commonly, epoxy resin, provides the compression strength and stabilizes the fibre component, like the concrete, and the fibre component, either glass or carbon, provides most of the tensile strength, more or less like the rebar in concrete.
Like a reinforced concrete structure, the problem becomes one of how to or orient the fibre component so that the fibres can continuously carry the tensile loads.   You can see right away that a resin compound by itself won't work for a high stress part; you have to have a tensile load bearing element embedded in the resin, and this load bearing element should be more or less continuous along the load path.
Random fibre segments in a resin matrix, like the chopped fibre fibreglass used in boats, won't do for something like a highly stressed beam.  The fibres have to be continuous from end to end, again, a lot like a reinforced concrete beam.  So this tends to rule out a process where a 3D printer could deposit resin and fibres at the same time.
It is possible to make certain aircraft parts from 3D printed plastic where the plastic by itself is replacing, say, an aluminum casting and the plastic resin is as strong, has the required hardness, and can handle the temperatures. Currently such parts would most likely be made by injection molding, 3D printing being so new.  But you will certainly see lower stress casting-equivalent parts start to emerge in aviation by 3D printing, especially for low volume parts where the process is just crying for a viable application and a certifiable process.  It's a conservative industry, so you have to give it time.
The challenge for now is how to make a resin matrix part, that needs high tensile strength, that can somehow be 3D printed with both the compression and tensile strength elements incorporated, and correctly oriented in the 3D print process.  Not so easy.
What will probably happen in the next 10 years is someone will come up with a radical new plastic compound incorporating something like graphene in it that has all the desired properties in all directions and can be machined from a block or deposited  and cured in a printing process.  Then we'll have 3D printed wing spars, frames and skins.

Even the compression strength is mainly delivered by the fibers. The polymer matrix only helps to keep the fibers in place (prevent buckling) and transfer loads into and out of the fibers by shear.

Yes where the fibres are straight as with unidirectional glass roving as used for spar caps and pull-truded products like Graphlite. Graphlite gets its incredible compression strength from having the carbon fibres perfectly straight so the resin has an easy job of stabilizing them. But I would say that in a component made from glass or carbon cloth where the fibres are wavy, already partly buckled you might say, not so much.

"The fibres have to be continuous from end to end, again, a lot like a reinforced concrete beam. So this tends to rule out a process where a 3D printer could deposit resin and fibres at the same time." You can buy 3D printers that lay down continuous carbon, glass, or Kevlar fibres end-to-end inside their filament.

Wow.  Well I guess it's coming down to development of a suitable resin that can be deposited, and size?

Check out Markforged. They build FDM printers that can also lay carbon fibers in the structure.

A short answer is that there are 3D printed planes in the RC model world.
However, they are heavier and more fragile than traditional materials, so they aren't common. What they are good at is producing a complex shape with lots of detail without expensive tooling.

Aviation is all about materials science
That is the only thing that kept Leonardo Da Vinci on the ground.  He was on the right track; If he had access to fiberglass-epoxy and a Lycoming engine, he would have had no trouble building an airplane.
Even in 1800 when the wood and sail technology was coming along, the metallurgy wasn't good enough to make an engine light enough. Stevenson was showing Watt that metallurgy was good enough to build smaller, faster 20 horsepower steam engines that could fit in a room instead of a house, but "faster" was a relative word.
The Achilles' heel of 3-D printing is materials strength.  That is why we don't 3-D print cylinder heads or hinges, and why it hasn't taken over the world.
3-D printing simply cannot accommodate aviaton-strength materials... Yet.
You could build an airplane, but to have enough strength to fly, it would be much too heavy to fly.

We actually do 3D print fuel nozzles for the world's most powerful commercial jet engine. GE also has a turboprop core that is constructed largely of 3D printed parts. Of course, to your answer's point, these are much fancier 3D printing materials than what you'll find in the average hobbyist's 3D printer.

Your last two paragraphs are way out of date. There are aviation strength materials that can be 3D printed and I’m not just talking a few gimmick parts. There are major structural components being printed for aero engines currently in development and near production readiness. That technology exists and is extremely close to being production level.

@Notts90 I don't disbelieve you, but keep in mind OP said "airplane" not "some components which lend themselves".  How far out are we from 3-D printing a wing or fuselage?  Or let's drop the difficulty a bit, how about a vertical stabilizer?  I am not naysaying you, I expect there may be a real answer there, and I would even suggest expanding it into an answer.

@Harper In my head I stuck the word “components” On the end as it made more sense than having a giant 3D printer. A key benefit of 3D printing in engines is being able to make complex parts that have historically been assembled from various components in a single piece. Can be stronger for less weight. I don’t mean normal things that happen to be in an engine, I mean major structural components specific to a particular engine and I believe blades will be next too. I’d write my own answer but it’s be too antidotal.

If we were to drop "plastic", the answer would be yes, Boeing is using 3D printing in the  787 Dreamliner. These are not plastic however but titanium. Somewhat stronger than your run-of-the-mill ABS plastic. From https://aerospaceamerica.aiaa.org/departments/making-3d-printed-parts-for-boeing-787s/

33-centimeter-long titanium fittings that anchor the floor of the aft kitchen galley to the 787 airframe and bear structural stresses

this also mentions the fuel nozzles of the GEnx engines are 3D printed by fusing metal powder with lasers.